Evaporation of spent liquors in pulp mills has been carried out by multi-stage evaporators operating by means of fresh vapor, spent vapor, vent gas or flash vapor. The number of stages refers to how many times the vapor boils the liquid before the last condensation. An evaporation unit is usually an apparatus with a separate system of conduits, e.g. heating elements in which vapor supplied to the vapor side condenses and is discharged as condensate. When the vapor condenses, it releases heat, which is conducted through a heat delivery surface to the liquid to be evaporated, which boils at its own boiling temperature, producing vapor. One of such evaporation unit types is a so-called falling film evaporator, in which black liquor is supplied in such a way that it flows down on the surface of the heating elements as a thin film, whereby a high heating effect is achieved. The purpose of evaporation is to remove water from the liquor that is evaporated in order to concentrate it. The operation of a multi-stage evaporation plant which evaporates liquor by fresh vapor or vent gas is based on the fact that energy supplied to the vapor side of the first stage as vapor, vent gas or in some other way boils the liquid on the liquor side, producing thus new vapor the temperature of which is lower than that of the substance producing heat energy, and the vapor thus produced has a lower pressure than the vapor supplied to the vapor side of the heat exchanger. The vapor produced is supplied to the vapor side of the following stage, i.e. the second stage. When the vapor condenses, it releases its heat to the liquor on the liquor side. The liquor heats up and boils, producing thus new vapor, the pressure and temperature of which are lower than those of the vapor supplied to this stage. This is repeated in the third stage, fourth stage, etc., until the pressure and temperature of the vapor discharged from the last stage are so low that technically and economically it is not worth continuing the process. The vapor produced is usually condensed with water or air in a condenser. The number of stages is limited by the pressure of the vapor that is used, the amount of vapor reserved for evaporation, and the size of the heat delivery surfaces of the units.
Since the temperature of the vapor supplied to the evaporation stages decreases constantly, the capacity of the evaporation plant is restricted. In order for the capacity to be increased, it is necessary at present to enlarge the heat delivery surface, i.e. to increase the number of evaporation pipes or elements, which increases the size of the evaporation unit in question. An alternative way to increase the capacity is to add new evaporation units or to change the operating principle of the evaporation units. All these solutions are rather expensive and usually require more space, which is not necessarily available without construction work.
European Patent No. 36 235 discloses an evaporator solution based on the thermo-compressor principle. The heating effect of an evaporation unit is increased by compressing vapor separated from the liquor by means of a vapor compressor to a higher pressure, and by recirculating this vapor to the evaporation unit in which it was separated. In this way, internal vapor circulation is provided in evaporators. According to the European patent, this principle can also be used by connecting a plurality of evaporation units in series, and by supplying the vapor separated in a stage with a lower pressure and temperature back to an evaporation unit having a higher pressure at the other end of the series after being compressed to a higher pressure by a compressor. The drawback of this solution is that, in addition to the necessary compressor, separate expensive evaporation units are required.
It is an object of the present invention to provide a method and arrangement by which the capacity of a multi-stage evaporation plant can be increased in a simple manner and at low costs. The method of the invention is characterized in that the pressure of the vapor which is discharged from at least one evaporation stage and which has been separated from the spent liquor in said stage is increased by a separate booster means, and that the vapor discharged from said booster means is supplied to the following evaporation stage for heating the spent liquor supplied thereto.